Tri - Band
Different cell phone companies use different frequencies in order to connect service subscribers. Some phones are programmed only to work on one or two frequencies. The type and amount of frequencies which you are able to operate on will affect the type of coverage that a phone subscriber can get.
There are four major frequencies which cell phones use to connect to the service provider's network. Frequency 850 and frequency 1900 are used in North America while frequencies 900 and 1800 connect phones in other parts of the world. Most phones purchased in North America will not be programmed to work outside of the 850 or 1900 frequencies. This means that those phones would become useless in other parts of the world.
Tri-band phones work on three frequencies, which means that a tri-band phone user can carry their phone with them overseas and usually find a connection on one of the other two frequencies.
By increasing the number of frequencies that your phone can operate on you will increase the number of countries in the world that your phone can work in.